An electric vehicle (EV), also referred to as an electric drive vehicle, uses an electric motor (or traction motor) for propulsion. Electric vehicles may include all-electric vehicles where the electric motor is the sole source of traction power, and hybrid electric vehicles that include an auxiliary power source in addition to the electric motor. In an electric vehicle, energy may be stored in one or more batteries to power the electric motor. When the stored energy decreases, the batteries may be recharged by connecting the vehicle to a power supply. Knowing the amount of energy left in a battery compared with the energy it had when it was full (called State of Charge or SOC) gives the user an indication of how much longer a battery will continue to perform before it needs recharging. Using the analogy of a fuel tank in a car, the state of charge provides similar information as the gas/fuel gauge in a vehicle.
Several methods of estimating the state of charge of a battery have been used. Most of these methods depend on measuring some convenient parameter which varies with the SOC. A current based SOC estimation technique, called coulomb counting, determines the remaining capacity in a battery by accumulating the current entering (charging) and leaving (discharging) the battery cells over time. A voltage based SOC estimation technique uses the voltage of the battery cell as the basis for calculating SOC. Battery manufacturers typically characterize the relationship between the open circuit voltage of the battery and its state of charge. Based on this relationship, the SOC may be determined by measuring its open circuit voltage.
Measuring the open circuit voltage, however, requires the battery to not be in use (i.e., without net current input or output) for a certain time before its voltage is measured. Determining SOC while the battery is in use is usually done by coulomb counting. However, errors in current estimation during prolonged charging events may lead to errors in the determined SOC. Embodiments of the present disclosure may alleviate the problems discussed above and/or other problems in the art. The scope of the present disclosure, however, is defined by the attached claims, and not by the ability to solve any specific problem.